Environmental Monitors on Lobster Traps

eMOLT Phase VI: Bottom Currents

After a few years of deploying surface drifters in the Gulf of Maine, we
started to think about the deeper, near-bottom currents. Since conventional
instrumentation was cost prohibitive and often difficult to use (given the
biofouling that occurs in our waters), we needed to devise a simple, less expensive
means of monitoring the flow. A new method suggested itself when the small
inclinometers came on the market. Could we secure a buoyant length of PVC
pipe to the ocean bottom, attach one of these $69 tilt meters, and infer the
flow?

Given the issue at that time associated with potential whale entanglements in lobster gear and the
uncertainty of whether sinking groundline is actually
needed in some regions, we finally proposed in 2007 to develop this low-cost method of
monitoring the bottom currents from lobster gear. As many lobstermen have
suggested, the existing groundlines may often lay down on
the seafloor given strong currents in certain areas. While some work was
currently underway to examine the degree to which groundline
is dragged down, little is known about the spatial and temporal variability of
bottom currents in these areas. As of this writing, we have tested and validated this idea and compared
the results with traditional current meters. It has worked exceedingly
well.

Beginning in the early summer of
2008, initial deployments were made by Vitalii Sheremet (lead developer) within the Woods Hole Oceanographic
Institute’s instrument test-well where tidal currents provide sufficiently-detectable
observations.Deployments of more traditional instrumentation were also be made
(alongside for comparisons) including two current meters: the Sontek Aquadopp owned by URI and the
Nortek Vector owned by NEFSC (see comparison in figure above).

After these initial multi-day
deployments in Woods Hole, we transferred the equipment to 10 local
lobstermen working along the New England coast.These individuals
have tested eMOLT equipment for us in the past. This
deployment was conducted over multiple weeks in an attempt to detect
variations due to storms and lunar periods. While the analysis of these
records are under way, an example result is pictured below and links to
other plots (as they are generated) are linked from Table 1 below.

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Given the recent advances towards a nation-wide ocean observing system, the idea is
ripe. New England lobstermen now have an
opportunity to make a significant contribution to the integrated system on the
Northeast Continental Shelf by securing an unprecedented array of relatively inexpensive sensors on
the seafloor. While the momentum is growing to build a large scale
integrated system, the funding is so far limited to existing infrastructure.
Until money is available to fund new initiatives, we are forced to take
small steps with low-cost alternatives. These small steps will, at least, provide some form of background data
in the intervening years and provide a system to gradually build in the future.

Much of our previous eMOLT work involved developing a network of participants. While this
outreach work (building relationships with fishermen) will certainly continue,
we hope to now focus the majority of our effort in providing useful products in
the form of web-served information. The results of our 2008 and 2010 pilot
studies are posted in Table 1. The figures below depict the
slight change in phototype with 2008 and 2010 on the left and right,
respectively. In the case of 2010, a secondary sensor is shown attached to
the base in order to monitor the orientation of the trap itself and
potentially document the trap orientation.

While the intial deployment in 2008 showed traps on their side or upside down nearly 25% of the time, the 2010 results reduce that percentage to only 6% of the time. There were only a few cases in 2010 where the trap WAS more than 45 degrees from level for greater than 25% of the time.
In some cases, where the tidal current is particularly strong or perhaps the groundline was overly taught, the trap was flipping on its side. In one month-long deployment in 86 meters, this happened a dozen times.
See zoomed-in figure below. It also happened in deep (274 meter) water near the Hudson Canyon.
A few dozen deployments were made in 2011 and all but one instrument
returned good data. Links to all the plots made for individual deployments
as provided in the tables below associated with each year.